1. Field of the Invention
The present invention generally relates to a device and a method for suppressing pulse interferences contained in a signal. As an example, the present invention relates to a device and a method for suppressing pulse interferences for a demodulator of COFDM type (“Coded Orthogonal Frequency Division Multiplex”).
2. Discussion of the Related Art
In COFDM modulation, data packets to be transmitted are put in the form of N complex coefficients associated with N respective frequencies (or carriers). Number N of the frequencies is equal, for example, to 1,705 for the so-called “2K” mode and to 6,817 for the so-called “8K” mode, in digital television radio transmission. The set of these N coefficients is processed by inverse fast Fourier transform (IFFT), which generates a “symbol” formed of a sum of modulated carriers, each carrier having an amplitude and a phase determined by the associated complex coefficient. The symbol thus generated is transmitted.
Conventionally, in radio transmission, the width of the information channel is 6, 7, or 8 MHz and each carrier is separated from the next one by a frequency difference Δf=1/Tu. Tu is the transmit time of a symbol and is called the operating lifetime. The operating lifetime is on the order of 224 μs in 2K mode and 896 μs in 8K mode, for a 8-MHz passband.
Upon reception, a receiver submits the symbol to the inverse processing, that is, mainly, a fast Fourier transform (FFT) to restore the initial complex coefficients. Certain carriers of the modulated signal, called pilot carriers, are used on demodulation to determine an estimate of the frequency response of the used information channel and to restore the fast Fourier transform of the modulated signal.
The signal received by the COFDM demodulator corresponds to the transmitted modulated signal modified by disturbances of various origins. It is thus necessary to correct the received signal to recover the transmitted signals. Among the disturbances affecting a received signal, the transmission noise and the pulse interferences can be distinguished. The transmission noise generally corresponds to a low-power disturbance which is permanently present and which is characteristic of the information channel taken by the signal between the receiver and the emitter. On the contrary, pulse interferences correspond to very short disturbances which however have a high power. Such disturbances are neither reproducible nor foreseeable, they are not present all the time, and may occur in isolated or grouped fashion at any time with an unforeseeable amplitude, duration, and starting time. Pulse interferences are, for example, due to electric equipments located close to the demodulator components.
The transmission noise and the pulse interferences can thus not be corrected with identical correction methods. The correction of the transmission noise generally uses the time stability properties of such a transmission. The correction of the pulse interferences generally implements a method consisting of individually detecting each pulse interference and of correcting the received modulated signal when a pulse interference is detected. An example of a conventional pulse interference detection and correction method is the zero substitution threshold detection method. Such a method is, for example, implemented on the demodulator referred to as STV0360, sold by STMicroelectronics. The case in point is to detect the presence of pulse interferences by comparing with a determined comparison threshold the amplitude of a digital modulated signal obtained by analog-to-digital conversion of the analog modulated signal received by the demodulator, generally after a frequency switching towards an intermediary frequency range. All the numerical values of the digital modulated signal greater than the determined threshold are replaced with a predetermined value, for example, the average value of the signal.
The main difficulty in threshold detection is the selection of the comparison threshold. Indeed, if the threshold is too high, only very high power pulse interferences are detected and pulse interferences with a lower power level cannot be detected. Conversely, if the detection threshold is too low, a portion of the digital modulated signal undisturbed by pulse interferences may be suppressed.